Semiconductor devices have become finer, and exposure light sources have shifted from g lines and i lines of conventional high-pressure mercury lamps to KrF and ArF lasers, which are called excimer lasers, having shorter wavelengths. Further, the use of an F2 laser, an electron beam, and X-rays has been examined.
Moreover, it is necessary to increase the NA (numerical aperture) of a projection lens to achieve a higher resolving power, thereby reducing a depth of focus. As is generally known, these relationships can be expressed by the equations below:(Resolving power)=k1(λ/NA)(Depth of focus)=±k2λ/NA2 where λ represents the wavelength of a light source used for exposure, NA represents the NA (numerical aperture) of the projection lens, and k1 and k2 represent coefficients associated with a process.
Meanwhile, a high resolving power and a high depth of focus are obtained by a phase shift mask, deformed illumination, and the like. However, methods using an F2 laser, an electron beam, and X-rays increase the cost of an apparatus. The phase shift mask or the deformed illumination, and the like, does not produce any effects in some circuit patterns.
Hence, attempts have been made to use a liquid immersion method, in which a projection exposure apparatus is provided with a nozzle having a liquid inlet surrounding the end of a projection lens (the closest lens to a wafer), a liquid is supplied through the inlet, the liquid is held between the projection lens and the wafer, and exposure is performed (see, for example, Japanese Patent Laid-Open No. 6-124873, the pamphlet of International Publication WO 99/049504, and Japanese Patent Laid-Open No. 10-303114).
Under the effect of liquid immersion, the above resolving power and depth of focus are represented by the equations below:(Resolving power)=k1(λ0/n)/NA0(Depth of focus)=±k2(λ0/n)/(NA0)2 where λ0 represents the wavelength of exposure light in the air, n represents an index of refraction of a liquid used for liquid immersion, a represents the half angle of convergence of a light beam, and NA0=sin α is established.
That is, the effect of liquid immersion is the same as the use of exposure light having a wavelength of 1/n. In other words, in the case of a projection optical system design having the same NA, liquid immersion can increase the depth of focus by n times. For example, when water is used as a liquid, n=1.33 is obtained, which means that the depth of focus is improved by 33%. Liquid immersion is effective for any patterns and can be combined with a phase shift mask method, a deformed illumination method, and the like.
The control performance of a stage in a liquid immersion state is changed and degraded relative to a state not having liquid immersion (hereinafter, referred to as a dry state) due to disturbances such as the surface tension, weight, and viscosity of a liquid. Hence, the alignment accuracy of the stage decreases. Such a phenomenon occurs statically and dynamically. Particularly, a step-and-scan exposure apparatus, which has become the mainstream is greatly affected by this phenomenon, because exposure is performed during driving. Moreover, in a local fill method, in which a wafer surface is partially brought to a liquid immersion state, when a shot on the outer periphery of the wafer is exposed, the wafer surface may be changed from a dry state to the liquid immersion state, and vice versa. In this case, a great disturbance is generated at the change between the liquid immersion state and the dry state.
When such a disturbance degrades the performance of the stage, it becomes impossible to obtain the greatest possible effect of liquid immersion exposure. As a matter of course, it is necessary to precisely control the vibrations, flow rate, thickness, volume, uniformity, temperature, and so on, of a liquid to solve this problem. The problem cannot be solved only by such control, and thus, measures using a stage control system are necessary.